Removal of sulfate from wet FGD wastewater by co-precipitation with calcium hydroxide and sodium aluminate.

Chemical precipitation method was adopted to remove sulfate from wet flue gas desulfurization (FGD) wastewater and mixtures of Ca(OH) (CH) and NaAlO (SA) were used as precipitants. The mechanisms of sulfate removal were explored according to the experimental and simulated results. These showed that three kinds of precipitations, which were gypsum, ettringite and co-precipitation onto aluminum hydroxides, were formed when sulfate in water reacted with CH and SA. The optimum operation condition for removing sulfate was that the molar ratio of CH/SA was 2, the initial pH value 5, the precipitant dosage 15 g/L, the reaction time 20 min, and the reaction temperature 55 °C. The sulfate was reduced from 4,881 mg/L to 784 mg/L under the optimized condition. In addition, the heavy metals and fluoride were also mostly removed. The post treatments of the supernatant illustrated that removal of sulfate from wet FGD wastewater by co-precipitation with CH and SA was a better choice.